Half of the world's population is infected with Helicobacter pylori, which causes chronic gastritis and gastric adenocarcinoma. Interventions based on high risk variables are needed. Antibiotics do not uniformly eradicate the infection, and benefits in reduction of gastric cancer in patients with precancerous lesions are not established. Our studies have directly implicated polyamines, derived from the rate-limiting enzyme ornithine decarboxylase (ODC), in gastric inflammation and carcinogenesis. We have reported that polyamines are increased in H. pylori gastritis in mice, and inhibition of ODC with alpha-difluoromethylornithine (DFMO) reduces gastric polyamines, and severity of H. pylori colonization and gastritis. In the gerbil model of gastric cancer, polyamine levels correlate with levels of gastritis, DNA damage, and progression to dysplasia/carcinoma, and DFMO suppresses polyamines and DNA damage, and reduces rates of dysplasia/carcinoma by more than 50%. We have demonstrated that the mechanism leading to H. pylori-induced DNA damage is induction of the enzyme spermine oxidase (SMO), which is downstream of ODC and generates H2O2 via metabolism of spermine. SMO expression increases along the histologic cascade from gastritis to precancerous intestinal metaplasia in North American subjects and in cases from Colombia and Honduras where H. pylori prevalence and gastric cancer rates are amongst the highest in the world. While inhibition of SMO also reduces cancer in gerbils, there are no suitable agents available for human use. Thus we will use SMO as a marker of risk, but focus on ODC as the target for chemoprevention, using DFMO, since there is more than two decades of experience in its use in human studies, including safety data and effectiveness in suppression of polyamine levels. ODC single nucleotide polymorphism (SNP) studies have predicted colon adenoma risk and response to DFMO, and we show that gastric cancer is associated with the GG allele of the ODC+316 SNP; thus we will examine the association of ODC SNP status with DFMO response. We have also found that DFMO reduces proinflammatory macrophage and T cell responses, and can have effects on H. pylori virulence both in vitro and in vivo. We hypothesize that high risk subjects with precancerous gastric lesions will benefit from DFMO treatment. This will include reduced oxidative stress associated DNA damage, inflammation, and bacterial virulence, leading to attenuated histopathology and cancer risk. Our Aims are to determine: 1.) The effect of DFMO in a clinical trial of 300 high risk subjects with precancerous lesions in Honduras and Colombia randomized to placebo or DFMO for 18 months, with assessment of DNA damage, gastric polyamines, and histopathology score. 2) Host factors associated with response to DFMO, including ODC SNP status, expression of SMO and other polyamine pathway and metabolic enzymes, and shift to immunotolerant immune responses. 3.) H. pylori bacterial factors, including strain genotypes and functional status of output strains. We expect our findings will set the stage for future longer-term studies of gastric cancer chemoprevention.